Silver halide photographic emulsions are manufactured by introducing the reagents--typically aqueous solutions of silver nitrate and a halide salt--into a reactor, where the fluid is well mixed such as by using a rotary agitator. The high level of mixing is accomplished by high speed stirring and turbulence in the fluid. The nucleation and the growth process, and hence the properties of the photographic emulsion grains, are directly affected by the extent of mixing in the reactor. Therefore, in order to minimize variability in the photographic performance of the emulsion, a high level of mixing is maintained throughout the time of the precipitation reaction. Additionally, emulsions are typically precipitated in the presence of a peptizer, which is usually gelatin, to maintain the colloidal stability of the particles. The use of high speed stirring results in the entrainment of air, which in the presence of gelatin leads to the formation of a stable foam. The volume of the foam continues to increase during the reaction, which is undesirable for several reasons. For one, the foam and the air bubbles interfere with the mixing conditions and can cause several dead reaction zones--leading to polydispersity in the properties of the resulting emulsion. The most serious problem of foam is that it occupies a significant volume in the reactor, which reduces the capacity of the reactor to produce a desired volume of the emulsion. Thus, foam generation directly affects productivity of a manufacturing operation.
In order to minimize problems encountered due to foam, some form of foam control is generally used. Chemical antifoamers that are added to the reactor can be classified as three distinct types: 1) defoamers that are added to break up a foam; 2) insoluble inorganic or organic materials; and 3) partially soluble or dispersible surface active materials.
Examples of defoamers of type 1) include alcohols such as butyl alcohol, octyl alcohol etc. The deficiency of these materials is that their antifoaming action is short term. That is, they are able to break the foam at the time of addition but cannot prevent subsequent formation of foam. Thus, they need to be added continuously resulting in large quantities of these materials associated with the emulsion. The presence of these materials can cause further problems in manufacturing operations where the emulsions are used, such as surface tension modification and vaporization. Thus, these materials are not generally desired.
Examples of insoluble organic materials of type 2) include silicone and paraffin oils. Frequently, these materials are made more effective by adding inorganic particles such as hydrophobic silica. These materials are quite effective at relatively low concentrations, and have a sustained antifoaming action. Also, these materials are non-volatile. This class of materials are disclosed in Research Disclosure 36929 (Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North Street, Emsworth, Hampshire P010 7DQ, ENGLAND). While these materials perform well as antifoamants in the reactor, they have catastrophic drawbacks in subsequent manufacturing operations, due to their interaction with other manufacturing hardware, such as UF membranes and filter membranes, which are a necessary part of the emulsion manufacturing process. Specifically, these insoluble materials foul ultrafiltration membranes which are used to deionize the emulsions, and they also plug filter media. Their most serious problem is that they are prone to form coating defects during the coating of solutions on photographic support. Creation of a large concentration of coating defects can make a product unusable. The details of the way these materials act to defoam as well as broad range of examples of these kinds of materials are given in "Defoaming", P. R. Garrett Ed., Surfactant Science Series, Vol. 45, Marcel Dekker, N.Y. 1993.
The third class of materials are partially soluble or dispersible surface active materials. These materials dissolve in the aqueous gelatin solution or disperse into very small drops, thereby minimizing the above mentioned problems. Examples of these materials include polyethylene oxide (EO)-polypropylene oxide (PO) block copolymers. U.S. Pat. Nos. 5,147,771, 5,147,772 and 5,147,773 disclose materials of this general class as grain growth modifiers to produce monodisperse emulsions. U.S. Pat. No. 5,587,282 discloses that among these materials, those having a PO content of 80% or more are effective as antifoamers. Other examples of these materials are disclosed in Res. Disc. 36929, as di and mono alkyl or alkenyl esters of polyethylene glycol having low water solubility. U.S. Pat. No. 5,587,282 also discloses polyalkylene oxide modified poly(dimethylsiloxane) fluids as antifoaming agents. All these examples fall under the general class of polyalkylene oxide containing organic materials with low solubility in water.
The general problem experienced by this class of materials is that, if the polyalkylene oxide content of the material is high, the potential of these materials to adsorb to the silver halide surfaces is high. This can result in grain growth modification as disclosed in U.S. Pat. Nos. 5,147,771, 5,147,772 and 5,147,773. While the use of growth modifiers for photographic emulsions may be useful in some instances, the growth modification property of this class of materials makes them unusable as a general purpose antifoamant. In many instances, the growth modification process which results from the interaction of the antifoamant with silver halide emulsion can be severe enough to induce agglomeration as disclosed in U.S. Pat. No. 5,681,692. It is generally believed that this occurs because the polyalkylene oxide, specifically polyethylene oxide, part of the molecule has specific interactions with the silver halide surface. This results in its being adsorbed in preference to the usual peptizing agent that is used, thereby reducing the colloidal stability of the grains. However, if the polyalkylene oxide content of the material is low, the solubility or dispersibility of these materials in the gelatin solution is not adequate. This results in large drops of the antifoamant being present in the emulsion. The large drops can cause filtration problems as well as coating defects. In addition, the size of the drops and the severity of the problems they can cause, is variable, as the formation of these drops depends on the stirring conditions of the reactor. Another problem is that these materials, being hydrophobic, may adhere to the surfaces of the hardware of the reactor.
Thus, it is desirable to have a material that acts as an antifoamant which can be dispersed in a reproducible manner, in the reactor, without interacting with the surfaces of the silver halide grains and the manufacturing hardware, so as to manufacture emulsions that have superior photographic performance.